Erosion is the process by which the surface of an object is worn away over time. The forces of erosion, such as those caused by sand, sun, water, and wind, can destroy the object. Airplane wings, helicopter rotor blades, boat hydrofoils and other foils experience especially strong erosion forces due to their movement through air or water. Erosion of these foils is a vexing and costly problem since damaged foils do not perform properly, and fixing or replacing them can be expensive and result in significant down time. It is difficult to predict the lifetime of a foil or when a foil should be replaced as different environments have significantly different effects on the erosion rates.
Various products have been developed that attempt to protect a foil surface from erosion forces, but these products suffer various limitations. Some commonly used current solutions, such as protective tape and protective boots (both available from 3M) are put on an airfoil surface to provide a barrier to erosion forces. A protective tape for an airfoil is generally a relatively thin, flexible material with a short, finite length (e.g., shaped like wide Scotch® tape) that is placed over part of a foil and held in place by an adhesive between the tape and the foil. Portions of the tape may sometimes be overlapped in order to adequately cover the surface. A boot has typically been preformed from a sheet of material into a specially shaped appliqué to fit specifically over part of the foil surface. Similar to the tape, it is generally held onto the airfoil surface with an adhesive. A protective tape or protective boot is typically applied to the leading edge of the airfoil, which experiences the most severe abrasive and wind forces, and extends partway along of the top side and bottom side of the airfoil. Commonly used adhesives for holding boots and tapes in place include pressure sensitive adhesives (PSA) and two-part epoxies. These boots, tapes and adhesives have various drawbacks.
The utility of boots and tapes is subject to the quality of application or installation and great care must be taken in preparing and installing them. Boots and tape can be dangerously unreliable if not installed properly. The usefulness of these products is very dependent on the nature of the substrate (the surface of the airfoil), to which they are applied. Different substrates (different surfaces of helicopter blades or airplane wings) have different inherent surface properties and have to be handled accordingly. A boot or tape requires a very clean surface in order to stick to a surface, and it may be difficult to get the surface sufficiently clean. Even small amounts of coatings or contaminants on a surface (especially near a tape or boot edge) can prevent proper adherence. A helicopter blade or airplane wing surface may have paint or another slippery coating or paint or a residual contaminant (such as hydraulic fluid, lubricant, silicone oil, other oils, etc.) on it that would prevent the tape or boot from sticking to it. The blade or wing surface may need to be specially prepped and contaminants may need to be completely removed with special chemicals, cleaners, or treatments. Proper installation if a boot or tape may require special equipment and a controlled, clean environment. Even a relatively small area of a tape or boot that is not adhered may cause problems, particularly if the poor adherence occurs along a boot or tape edge or other high stress area. In addition to unclean or slippery areas, air bubbles may form during installation and prevent the boot or tape from sticking to the surface. Boot or tape installation that proceeds without proper surface preparation may cause boot or tape failure with disastrous results. Airfoils are subject to tremendous wind forces, and it is very dangerous if a protective material such as a boot or tape becomes loose and falls off mid-flight. Even beyond airfoil damage due to erosion, a boot or tape that falls even partially off a helicopter blade may get caught in or otherwise damage a helicopter rotor or tail rotor or even stop one of the rotors altogether, interrupting flight or even bringing down the entire helicopter.
Once in place, a boot or tape needs to not interfere with flight aerodynamics. In particular, tapes and boots need to fit closely on the airfoil during normal use and maintain the aerodynamic properties of the helicopter or airplane. Once a tape or boot is in place on a foil, the tape or boot material and the adhesive start to age. Adhesives are temperature dependent and decay over time, but it is difficult to predict exactly when that will be. Physical properties of an adhesive, such as peel strength and shear strength, are greatly affected by both hot and cold temperatures. Commercial and military aircraft are used in harsh environments all over the world and often operate at both ends of the temperature spectrum in a single day, subjecting the adhesives and protective materials to great stresses. Boots and tapes have edges (which are typically found on the trailing edge of a foil) that can disrupt airflow and change flight behavior. These edges are also vulnerable to getting pulled on and pried up by tremendous wind forces when an adhesive starts to fail, dislodging the boot or tape from the foil mid-flight. A whole section of tape or boot may be pulled away. Even detaching a small area of tape from a helicopter blade can lead to severe vibration and dangerous consequences for the helicopter and pilot. Tape that completely or partially detaches from a helicopter blade can becomes entangled in the helicopter tail rotor, and may cause a crash. In a combat situation, if the occupants are lucky enough to survive the crash, being stranded behind enemy lines is equally life threatening.
Boots and tapes wear out over time and can be difficult to remove and replace. As mentioned above, the quality of boot and tape installation is highly dependent on the airfoil surface and the quality and nature of the surface changes over time. An unprotected blade surface or a protective boot or tape on an airfoil is damaged by abrasion and wear from sand and pebbles, aging and regular use, corrosion from salt water, damage from weapons, oxidation damage, ultraviolet (UV) light damage from the sun, or weathering and cracking due to heat and cold exposure. A tape or boot may not fit well onto a damaged blade surface. The foil surface may have, in addition to coatings, paint, and contaminants that can interfere, bits of an old boot or tape stuck to it. Erosion may have left behind only adhesive or may have broken the tape or boot down into numerous small pieces of adhered film; these small bits can be very difficult to remove. For optimal performance, all of a used tape or boot has to be completely peeled off a blade before a new one is installed. Removal may require special equipment, specific cleaners, or a special (clean) environment. Cleaning all tape and residue from the surface may require use of a wallpaper streamer or a sander or other abrasive methods and the cleaning and sanding processes may further damage the blade. Relying on an adhesive to secure a protective layer to an airfoil is not an optimal solution.
Abrasion strips made of metal, such as nickel, stainless steel or titanium, are also sometimes used on helicopter blades to protect the blades from erosion. These are typically applied to the leading edge of an airfoil, generally covering the first 15% of the chord length on the top and bottom. In the highest wear areas, a metallic cap, such as electroformed nickel that is tapered to a thin trailing edge so as to reduce impact to airfoil performance is bonded on top of an abrasion strip. In addition to many of the problems described above with adhering boots and tapes to a foil, metallic abrasion strips are softer than sand, and when a helicopter blade is hit by sand, the sand can rapidly erode the metal strip. The erosion of these strips is especially noticeable on the outboard sections of the blade where tip velocity approaches Mach 1. Additionally, as documented during combat operations in Afghanistan, the erosion of a metal strip caused by powerful and rapidly moving sand can cause the formation of a visible halo or corona around the rotating helicopter blade. This corona effect, as it is called, is thought to occur due to static discharge or to the creation of a cloud of small metal particulates that ignite, causing a spontaneous and visible spark due to pyrophoric oxidation. The highly visible halo is spectacularly and undesirably visible in the dark at night as a large, bright circle. The corona effect makes the helicopter visible from a great distance which is particularly detrimental for a helicopter that needs to be stealthy.
Spray coatings are also sometimes used to protect helicopter blades. As with the tapes and boots, careful surface preparation is critical. A helicopter blade may need to be removed from the helicopter for spray coating installation and hung in a special spray booth. The blade has to be carefully cleaned in order for the spray coating to stick to the surface. A spray coating is generally applied to a blade in a liquid form in the spray booth. Areas of the blade that should not be coated have to be protected or masked to prevent coating. Multiple rounds of coating and drying are generally required. Special controls are required to ensure that the coating is applied with a unifomi thickness. Once applied, these coatings may be thin and wear away quickly and need frequent replacement. They may provide inadequate protection against harsher environments or objects. Using such coatings reduces operational readiness and allows fewer missions to be run.
Thus there is need for improved methods to protect an airfoil, hydrofoil, or other foil and to extend the life of these foil surfaces. Provided herein are articles, materials, and methods for protecting moving parts from degradation, such as by protecting an airfoil or hydrofoil from erosion forces from sand, sun, water, or wind.